This invention relates to a method for mounting up a plastic snap fastener element comprised of a base with a central aperture and of a post having a stem adapted to pierce through a supporting sheet piece and to extend through the aperture in the base.
As a method for mounting up a snap fastener of the type described, it has conventionally been well known, as per FIG. 5 of the accompanying drawings given to be referenced for this purpose, to prepare the stem 6 with an open-tipped axial central bore of a small diameter 8 formed for facilitating plastic deformation of the stem 6, so as to force the same to pierce through the supporting piece A and extend through the aperture 4 in the base 1, to heat and soften the tubular stem 6 in such state and to crush the same by axial pressure while thereby causing the same to expand radially outwardly; but such has been accompanied by serious drawbacks with respect to the efficiency of the mounting work as well as the resulting strength of the mounted assembly.
Namely, it takes a long time since the tubular stem 6 is first heated and softened and is them crushed under pressure for causing the mounting, and accordingly the efficiency of the mounting work is quite low.
On the other hand, if the process of forcing the stem 6 to pierce through the supporting piece and crushing the same is repeated at a high speed such as tens of or hundreds of times per minute, then each stem 6 can hardly be heated and softened in the short retaining time allotted thereto, even though the crushing tool, such as a hammer or the like, is equipped with heater means incorporated therein for the purpose of the heating and softening. The stem 6 will therefore substantially in a cold process be crushed under axial pressure while the open hollow tip brim thereof is thereby expanded radially outwardly, and as the outward expansion proceeds there will inevitably occur cracks in the said brim portion so that the closure head formed thereon by the crushing deformation in clamping engagement with edges around the aperture 4 will be in a petaloid or stellate shape, whose projections are apt to chip off, resulting in failure of sufficient clamping strength of the mounted assembly. Specifically, when the supporting sheet piece A is a woven fabric as is normally the case in clothes, walls of the tubular stem 6 must be sufficiently thick in order to make the stem rigid enough to forcibly pierce through the fabric, then however the stem 6 has the outer diameter too large to anyhow push its way through the fabric mesh space forcibly widening same by shoving aside the woven threads, and so the threads of the fabric come inevitably in the way as the stem 6 is forced to pierce through the fabric, causing danger of splitting thereby the tubular stem 6. On account thereof, it is required to make the walls of the tubular stem 6 as thin as possible and to make tip ends thereof sharply pointed, but the thinner the walls, the weaker will be the clamping strength as is attained by the petaloid or stellate closure head formed by the crushing deformation as described hereinabove in clamping engagement with edges around the aperture 4, as may thus result in such poor mounted assembly of the base 1 and the post 5 as are apt to be separated from each other off the assembly by stresses as are caused when the clothes are washed, as will thus substantially impractical.